Improvements in gas compressors have been occurring for decades, and various types of cost-effective gas compressors are manufactured today that are suitable for obtaining compression ratios of 10:1 or less. In many applications, those relatively low compression ratios are satisfactory, and if higher compression pressures are desired, conventional compressors may be placed in series. These conventional compressors generally tend to intermittently increase the temperature of the gas being compressed, but the temperature of both the compressor and the gas may be maintained within acceptable limits due to the low compression ratios.
In other applications, compression ratios greater than 10:1 are desired from a single compressor. Conventional compressors are frequently bulky, and placing compressors in a series to obtain a desired pressure may not be practical due to size and/or weight limitations. In outer space applications, for example, high gas pressures are desirably output from a relatively small and light-weight compressor. In other applications, the operating temperature of the compressor components and/or the gas being compressed must be carefully controlled, even when a high compression ratio is desired. When oxygen is being compressed, for example, its temperature must be carefully regulated throughout the compression cycle to ensure safety.
Compressors may be initially classified as a function of their driving source. Mechanically driven compressors include a reciprocating rod to drive a piston with respect to a cylinder, although the rod itself may be powered or moved from any number of conventional electric, hydraulic, or mechanical power sources. Fluid-driven compressors, on the other hand, generally drive a piston with respect to a compressor cylinder by fluctuating the liquid pressure acting on the face of the piston. Fluid-powered compressors are frequently connected to a pressurized hydraulic source, and are sometimes referred to as being hydraulically powered compressors. Although various gases or liquids may be used to reciprocate the piston with respect to the cylinder, oil is a preferred hydraulic fluid for many applications. Hydraulically powered compressors are desired for many applications, since a hydraulic power supply may otherwise be present at a plant, job site, spacecraft, or other location desiring compressed gas, so that a separate source for powering the compressor need not be provided. For many applications, fluid-driven compressors thus provide substantially increased versatility and portability over mechanically driven compressors, which generally require a separate power source.
Hydraulically powered gas compressors may be generally classified as (1) diaphragm compressors, (2) rotary compressors, and (3) piston compressors. Diaphragm compressors utilize a diaphragm that flexes within the elastic limit of the diaphragm material in response to a change in fluid pressure on one side of the diaphragm to compress a gas on the other side of the diaphragm. Conventional suction and exhaust check valves are utilized to pass the gas through the compressor, and the compressor stroke is relatively low due to the necessity to remain within the elastic limit of the diaphragm. Diaphragm compressors have a relatively large compressor hardware-surface to gas-volume ratio, which makes the compressors well suited for maintaining both the compressed gas and the compressor components within acceptable temperature limits. While compressor displacement can be increased by increasing the diameter of the diaphragm, large displacement diaphragm compressors become very massive and impractical. The diaphragm itself is comparatively short-lived due to stresses imposed on the diaphragm during each compression cycle as it flexes to displace the gas. Rotary compressors (sometimes called blowers) have high volumeric through-put, but like diaphragm compressors have comparatively low compression ratios. Neither diaphragm compressors nor rotary compressors are thus generally suitable for generating compression ratios greater than 10:1.
Conventional piston compressors are similar in configuration to an internal combustion engine, although movement of the piston is used to compress a gas rather than to power a rod and rotate a shaft. Although there is no combustion process occurring in a gas compressor, heat is nevertheless generated due to the adiabatic compression of the gas to a higher pressure state. In relatively low ratio compressors, cooling is conventionally provided for the compressor cylinder and head, but not for the compressor piston. In order to minimize peak gas temperature in conventional piston compressors, the compression ratio is thus generally maintained at 10:1 or less for any particular piston and cylinder arrangement, and multiple stages or series arrangements with intercooling may be used to achieve higher overall compression ratios. When larger displacements or higher compression ratios are attempted with conventional piston compressors designs, the increase in the cylinder diameter or piston stroke causes the temperature of the core gas at the geometric center of the gas volume to substantially increase. Since gas at this geometric center of the gas volume is spaced further from the piston, cylinder wall, and head surfaces than gas elsewhere in the compression chamber, this gas is cooled less than gas outward of this geometric center. Increased temperature of the core gas allows undesirable chemical reactions and decomposition of the compressor materials, which destroys the compressor. Also, the increase of the core gas temperature may result in unsafe and/or undesirable reactions of the gas being compressed. Accordingly, compression ratios for conventional piston compressors are maintained at safe levels generally below 10:1, while cooling is provided primarily for the compressor cylinder and head.
People familiar with piston gas compressors have recognized their significant limitations for over a century. U.S. Pat. No. 129,631 to Waring discloses a double-acting piston and cylinder compressor, which has a reciprocating drive shaft powered by an external source and a disk-shaped hollow piston moved by the shaft within the cylinder. Suction and discharge valves are located at the cylinder ends, and the compressor is internally and externally cooled and lubricated by water or other coolant. U.S. Pat. No. 2,211,029 to Robinson discloses a pump with a hollow piston for piston cooling. U.S. Pat. No. 3,256,835 to Kraus discloses a hand operated pump with specialty valving to regulate fluid flow. None of these patents disclose a hydraulically driven gas compressor.
Various attempts have been made to devise gas compressors that avoid some of the prior art problems. One design concept utilizes a compressor with a stationary piston and a sliding cylinder, rather than a stationary cylinder and reciprocating piston. U.S. Pat. No. 2,042,673 to Maniscalco discloses a compressor with a plurality of sliding cylinders operating at multiple stages, with a stationary piston in each stage. The cylinders are mechanically driven, and the piston is internally cooled by a coolant, although in this case the cylinders are not liquid cooled. U.S. Pat. No. 2,152,054 to Johnson discloses a similar gas compressor that has two sliding cylinders operating as first and second stages about respective stationary first and second pistons. The cylinders are mechanically driven. The first stage piston is cooled by a flowing coolant, although the cylinders are not cooled. None of these patents discloses a hydraulically driven gas compressor. Devices of this latter type are not particularly practical for many compressor applications, and gas compressors with stationary pistons and sliding cylinders have had little marketplace acceptance.
The disadvantages of prior art gas compressors are overcome by the present invention, and an improved gas compressor is hereinafter disclosed. The gas compressor of the present invention is hydraulically driven to reciprocate a spool-type piston within a compressor cylinder, and both the cylinder and piston may be cooled to achieve reliable operation while providing for relatively high compression ratios. The gas compressor of the present invention is relatively simple in concept and thus reliable, yet its design allows for easy modification so that the compressor size and piston stroke can be easily regulated to match a particular application.